Recent studies strongly suggest that there are common, genetically determined pathways to risk for psychiatric and neurodevelopmental disorders including autism, intellectual disability, attention deficit disorder, and schizophrenia, but no study has investigated the relationship between genetic variation and human brain development prior to the age at which clinical symptoms are first recognized. The primary objective of the current application is to use cutting-edge techniques in genomics to identify common and rare genetic variants which impact brain development in the early postnatal period, an extremely dynamic time which may be critical in the etiology of neurodevelopmental disorders. Intracranial volume (ICV), total white matter, total gray matter, lateral ventricle volume, and maturation of white matter tracts are heritable in neonates. The proposed project will test several major genetic mechanisms which could explain this high heritability. (1) We will test whether variation in structural brain phenotypes is predicted by common variants of moderate to large effect size by genotyping approximately 1 million single nucleotide polymorphisms (SNPs) and a genome-wide set of copy number variation (CNV) probes in a large (900 subject) and well-characterized population sample of children assessed with high-resolution MRI of the brain at 2 weeks of age with T1- weighted, T2-weighted, and diffusion tensor imaging sequences. Analysis will be completed in 2 stages, a hypothesis driven test of a defined set of genetic variants previously implicated in brain development and a hypothesis-generating unbiased search of the genome to identify previously unsuspected variants affecting brain development. (2) Using the same data set we will test whether variation in structural brain phenotypes is predicted by the combined effects of many common variants each with a small effect size through pathway analysis and advanced multi-marker association models. (3) We will also use this data set to test if the total burden of rare genic CNVs predicts variation in brain structure. (4) We will test whether rare SNPs and/or small insertions and deletions are associated with brain development by performing full exome sequencing in a subgroup of 20 children with enlarged ventricles, a phenotype which is highly relevant to neurodevelopmental disorders. While the focus of this grant is on the neonatal period, participants are also returning for follow-up scans and detailed developmental assessments at 1, 2, 4 and 6 yrs of age as part of 2 already funded studies. Thus, ultimately, the information generated in this grant can be used to study genetic determinates of the trajectories of structural and functional brain development across the critical transitional period of infancy and early childhood. This is an unprecedented opportunity to identify genetic variants which impact brain development, potentially mediating risk for psychiatric and neurodevelopmental disorders. A better understanding of such genetic mechanisms has the potential to inspire new approaches to prevention, diagnosis, and treatment which are urgently needed.